Three conventional processes are primarily known for synthesizing a quaternary ammonium salt of an organic acid. They are as follows:
(1) Preparing a quaternary ammonium salt of an organic acid by reacting a halogenated quaternary ammonium salt, which has been synthesized by a quaternizing reaction of a tertiary amine with an alkyl halide, with a metal salt of an organic acid in a solvent, and then removing the insoluble halogenated metal salt JP-A-63-8359 ("JP-A" as used herein means an "unexamined published Japanese patent application"). EQU R.sub.3 N+RX.fwdarw.R.sub.4 NX EQU R.sub.4 NX+MY.fwdarw.R.sub.4 NY+MX PA1 (2) Preparing a quaternary ammonium salt of an organic acid by first synthesizing a quaternary ammonium hydroxide by subjecting a halogenated quaternary ammonium salt, which has been synthesized through a quaternizing reaction of a tertiary amine with an alkyl halide, to form a quaternary ammonium hydroxide by electrolysis (JP-B-45-28564; "JP-B" means an "examined Japanese patent publication"), by ion exchange resin (JP-A-52-3009), by the silver oxide method (R. C. Peterson et al., J. Amer. Chem. Soc., 1959, 81, 3264) or the like, and then neutralizing the resulting ammonium salt with an organic acid. EQU R.sub.4 NX.fwdarw.R.sub.4 NOH EQU R.sub.4 NOH+HY.fwdarw.R.sub.4 NY+H.sub.2 O PA1 (3) Preparing a quaternary ammonium salt of an organic acid by reacting a quaternary ammonium carbonate, which has been synthesized by quaternizing a tertiary amine with a carbonate diester, with an organic acid and then subjecting the reaction mixture to decarboxylation (JP-A-63-280045). EQU R.sub.3 N+R.sub.2 CO.sub.3 .fwdarw.R.sub.4 NCO.sub.3 R EQU R.sub.4 NCO.sub.3 R+HY.fwdarw.R.sub.4 NY+ROH+CO.sub.2
In the first process (1) described above, a quaternary ammonium salt is separated by making use of the difference in the solubility between the quaternary ammonium salt of an organic acid and a metal halide salt. This method is accompanied with the drawback that the metal halide salt cannot be removed from the reaction mixture completely so that only a low-purity quaternary ammonium salt of an organic acid can be obtained.
The second process (2) described above, which employs a quaternary ammonium hydroxide is one of the most commonly used synthesis processes. For industrial preparation, this process is carried out by the electrolysis method. However, preparation of a high-purity product having impurity contents which are on the order of ppm is costly. For preparation on the laboratory scale, the ion exchange resin method or silver oxide method is normally used. However, both of these techniques present problems in purity similar to the process (1) described above.
Process (3) is the most advantageous process of the three processes. However, from the viewpoint of the purity of the quaternary ammonium salt, the process requires high-temperature and high-pressure reaction conditions.
For the preparation of an N-alkyl-N'-methylimidazolinium salt of an organic acid by using an N-alkylimidazoline as a tertiary amine and adopting the above-described synthesis of a quaternary ammonium salt of an organic acid, neither process (1) nor process (2) can be employed, because the former cannot provide a high purity product and the latter does not permit the stable existence of a hydroxide of an N-alkyl-N'-methylimidazolinium (B. Fernandez et al., J. C. S. Perkin II; 1978, 545).
According to process (3), it is impossible to obtain a high-purity N-alkyl-N'-methylimidazolinium salt of an organic acid in a high yield, because, different from the conventionally-employed tetraalkylammonium methyl carbonate, N-alkyl-N'-methylimidazolinium methyl carbonate formed in the quaternizing reaction step cannot be isolated because of its thermal instability and moreover it inevitably undergoes decomposition prior to reaction with an organic acid.
Furthermore, since an N-alkylimidazoline as the raw material and N-alkyl-N'-methylimidazolinium methyl carbonate are unstable to water and undergo hydrolysis as shown in Formulae (2) and (3), respectively, it is impossible to obtain a high-purity N-alkyl-N'-methylimidazolinium salt of an organic acid in a high yield. ##STR1##
In the above-described processes for producing an N-alkyl-N'-methylimidazolinium salt of an organic acid, N-alkyl-N'-methylimidazolinium methyl carbonate, which is formed in the quaternizing reaction step, is thermally unstable and is prone to undergo hydrolysis. It is impossible to apply the conventional process for the preparation of a quaternary ammonium salt of an organic acid as disclosed in JP-A-63-280045 without any modification. A need, therefore, continues to exist for a process of preparing an N-alkyl-N'-methylimidazolinium salt of an organic acid in high yield and high purity.